How poverty passes from generation to generation is now becoming clearer. The answer lies in the effect of stress on two particular parts of the brain

THAT the children of the poor underachieve in later life, and thus remain poor themselves, is one of the enduring problems of society. Sociologists have studied and described it. Socialists have tried to abolish it by dictatorship and central planning. Liberals have preferred democracy and opportunity. But nobody has truly understood what causes it. Until, perhaps, now.

The crucial breakthrough was made three years ago, when Martha Farah of the University of Pennsylvania showed that the working memories of children who have been raised in poverty have smaller capacities than those of middle-class children. Working memory is the ability to hold bits of information in the brain for current use—the digits of a phone number, for example. It is crucial for comprehending languages, for reading and for solving problems. Entry into the working memory is also a prerequisite for something to be learnt permanently as part of declarative memory—the stuff a person knows explicitly, like the dates of famous battles, rather than what he knows implicitly, like how to ride a bicycle.

Since Dr Farah's discovery, Gary Evans and Michelle Schamberg of Cornell University have studied the phenomenon in more detail. As they report in this week's Proceedings of the National Academy of Sciences, they have found that the reduced capacity of the memories of the poor is almost certainly the result of stress affecting the way that childish brains develop.

Dr Evans's and Dr Schamberg's volunteers were 195 participants in a long-term sociological and medical study that Dr Evans is carrying out in New York state. At the time, the participants were 17 years old. All are white, and the numbers of men and women are about equal.

Stress in the city

To measure the amount of stress an individual had suffered over the course of his life, the two researchers used an index known as allostatic load. This is a combination of the values of six variables: diastolic and systolic blood pressure; the concentrations of three stress-related hormones; and the body-mass index, a measure of obesity. For all six, a higher value indicates a more stressful life; and for all six, the values were higher, on average, in poor children than in those who were middle class. Moreover, because Dr Evans's wider study had followed the participants from birth, the two researchers were able to estimate what proportion of each child's life had been spent in poverty. That more precise figure, too, was correlated with the allostatic load.

The capacity of a 17-year-old's working memory was also correlated with allostatic load. Those who had spent their whole lives in poverty could hold an average of 8.5 items in their memory at any time. Those brought up in a middle-class family could manage 9.4, and those whose economic and social experiences had been mixed were in the middle.

These two correlations do not by themselves prove that chronic stress damages the memory, but Dr Evans and Dr Schamberg then applied a statistical technique called hierarchical regression to the results. They were able to use this to remove the effect of allostatic load on the relationship between poverty and memory discovered originally by Dr Farah. When they did so, that relationship disappeared. In other words, the diminution of memory in the poorer members of their study was entirely explained by stress, rather than by any more general aspect of poverty.

To confirm this result, the researchers also looked at characteristics such as each participant's birthweight, his mother's age when she gave birth, the mother's level of education and her marital status, all of which differ, on average, between the poor and the middle classes. None of these characteristics had any effect. Nor did a mother's own stress levels.

That stress, and stress alone, is responsible for damaging the working memories of poor children thus looks like a strong hypothesis. It is also backed up by work done on both people and laboratory animals, which shows that stress changes the activity of neurotransmitters, the chemicals that carry signals from one nerve cell to another in the brain. Stress also suppresses the generation of new nerve cells in the brain, and causes the “remodelling” of existing ones. Most significantly of all, it shrinks the volume of the prefrontal cortex and the hippocampus. These are the parts of the brain most closely associated with working memory.

Children with stressed lives, then, find it harder to learn. Put pejoratively, they are stupider. It is not surprising that they do less well at school, end up poor as adults and often visit the same circumstances on their own children.

Dr Evans's and Dr Schamberg's study does not examine the nature of the stress that the children of the poor are exposed to, but it is now well established that poor adults live stressful lives, and not just for the obvious reason that poverty brings uncertainty about the future. The main reason poor people are stressed is that they are at the bottom of the social heap as well as the financial one.

Sir Michael Marmot, of University College London, and his intellectual successors have shown repeatedly that people at the bottom of social hierarchies experience much more stress in their daily lives than those at the top—and suffer the consequences in their health. Even quite young children are socially sensitive beings and aware of such things.

So, it may not be necessary to look any further than their place in the pecking order to explain what Dr Evans and Dr Schamberg have discovered in their research into the children of the poor. The Bible says, “the poor you will always have with you.” Dr Evans and Dr Schamberg may have provided an important part of the explanation why.